The Duke and Duchess of Cambridge don't need to put on a public display of affection when they're on royal engagements because they're a strong couple, a body expert has claimed.

The royal couple - who have four-year-old son Prince George, two-year-old daughter Princess Charlotte and another little one on the way - have rarely been seen showing public displays of affection and, according to a body expert, it's because they don't like to parade their love while they're both on official royal engagements.

Robin Kermode told the MailOnline: "It is very clear that William and Catherine are emotionally close, however, they do not seem to feel the need to prove their love - particularly when on official state business.

"They come across as a strong, independent couple and while they are always on show, they do not feel the need to 'show' their love for each other to the world."

However, William's brother Prince Harry has taken a different approach with his girlfriend Meghan Markle as they couldn't keep their hands off one another last month when they watched the Invictus Games together in Toronto, Canada.

Although the pair are free to do as they please when the 33-year-old flame-haired hunk isn't working, it's believed their behaviour will change once the 'Suits' actress, 36, marries into the royal family because they'll both be on "duty."

Meanwhile, the couple - who have been dating just over a year - have been rumoured to be getting engaged for months now, but friends believe it's only a matter of time before Harry makes the announcement that they're set to get hitched.

A source said: "While Meghan may not be wearing a ring or a formal engagement announced, it's fair to say they're as good as engaged now.

"They've spoken about their marriage plans openly with each other, and friends and family around them are pretty much thinking about wedding attire already!"